Friction stir welding is a known technique for welding together workpieces across a joint therebetween. The process is described in U.S. Pat. No. 5,460,317, the disclosure of which is incorporated herein by reference, and generally comprises passing a rotating stir pin between the workpieces at the joint so that the stir pin frictionally heats and plasticizes the material of both workpieces surrounding the stir pin and causes the plastic material to mix, and advancing the rotating stir pin along the joint to metallurgically bond the workpieces together.
Friction stir welding has provided a practical process for welding certain materials which theretofore could not practically be welded to create structural assemblies. For example, precipitation-hardened high-strength aluminum alloys are attractive candidates for constructing high-performance structures having high strength-to-weight ratios, but prior to the development of the friction stir welding method their full advantage could not be realized in structures having linear welded joints because no process for effectively weld such materials along linear welds had been available. Consequently, mechanical fasteners had to be used for joining components formed of high-strength aluminum alloys. The added weight of the fasteners compromises the weight-reducing benefits of these high-strength materials.
With the advent of the friction stir welding process, a method for effectively joining these high-strength aluminum alloys along linear joints without mechanical fasteners has been provided, thereby removing one of the barriers to more-widespread use of such materials for constructing high-performance structures. However, another barrier to their use still remains. Specifically, the carefully engineered, corrosion-resistant microstructure of high-strength precipitation-hardened aluminum alloys tends to become altered in the process of being welded, such that the weld zone is prone to corrosive degradation at an accelerated rate compared to the material outside the weld zone. The result is welded joints having poor corrosion resistance. Although welded structures are typically chemically treated to enhance their corrosion resistance prior to being placed into service, this type of treatment would be inadequate to overcome the poor corrosion resistance of the welded joints.
Thus, prior to the present invention there has not been available a technique for effectively welding high-strength aluminum alloys and the like while preserving adequate corrosion resistance at the welded joints.